This invention relates to an X-ray generator with current measuring device and more particularly to a current measuring device for use with an X-ray generator suitable for measuring currents of an X-ray tube accurately and readily under electrical insulation.
According to "General View of Medical Radiation Appliances Technology in 1984" edited by The Institute of Radiation Appliances Industry, a corporate juridical person, pp 251-254, the conventional method of measuring X-ray tube current falls into measurement at a neutral point and measurement at a high voltage point of the high voltage circuit. In the former, the neutral point of output voltage from the power supply unit feeding high voltage to the X-ray tube is grounded and an output current of the power supply unit is detected near the neutral point as a voltage drop across a shunt resistor. In X-ray generators, since the X-ray tube is applied with a maximum of about 150 KV of high voltage, the high voltage cable is used to connect the power supply unit to the X-ray tube. Current charging the high voltage cable adds to the X-ray tube current and a resultant current, amounting to a peak value which is ten times as large as that of the X-ray tube current, comes into the neutral point. The output current from the power supply unit detected at the neutral point therefore contains a large error factor due to the charging current. This disadvantageously prevents accurate measurement of the X-ray tube current. Moreover, noise and surge generated in the high voltage circuit passes through the shunt resistor to interfere with the X-ray control unit, causing the X-ray generator to operate erroneously.
In the latter measurement of the X-ray tube current at the high voltage circuit point, the X-ray tube current per se can be measured accurately but because of the high voltage, 150 KV in the extreme, applied to the high voltage circuit at which the measurement is implemented, the X-ray tube current must obviously be measured by means of an instrument electrically insulated from the high voltage circuit. For measurement under electrical insulation, there are known and available typical types of measurement based on photoelectric conversion and magneto-electro conversion. A measuring instrument of either type is inserted into the high voltage circuit and therefore its structure is necessarily complicated and can be obtained only through a sophisticated expensive manfacturing process. Accordingly, such a measuring instrument may be employed for inspection or evaluation of the X-ray generator but it can not be a commercial product which can conveniently be incorporated into the X-ray generator to participate in continuous measurement and control of the X-ray tube current.
Japanese Utility Model Unexamined Publication No. 60-175499 discloses that the current of an X-ray tube can be measured by using a magnetic sensor such as a hall element at the high-voltage cable of the X-ray device.